1. Field of the Invention
This invention relates to a method for degrading or depolymerizing non-thermosetting, i.e., meltable, organic polymers. More particularly, the method comprises the thermal oxidative degradation of molten polymers by sparging them with small, oxygen-containing, gas bubbles of a particular size in the absence of a catalyst. The method is particularly useful for the treatment of amorphous polypropylene, homopolymers and copolymers of olefins, and synthetic waxes prepared by the Fischer-Tropsch process.
2. Description of the Prior Art
By way of background, organic polymeric chemicals provide an exceptionally complicated variety of chemical and stereochemical configurations. The chemical literature is replete with references concerning the production of such polymers and their treatment in order to alter their diverse physical and chemical properties for various industrial purposes. The polymerization reaction of simple alkene and conjugated dienes produces many useful substances of commerce. Numerous examples of polymerization exist, such as, the treatment of ethylene in the presence of oxygen, heat, pressure and catalysts to produce polyethylene, the polymerization of vinyl chloride to polyvinyl chloride, etc. Some of these polymerizations can be designed to produce stereospecific polymers. Polypropylene, for example, can exist in a high melting crystalline form, a soft and rubbery amorphous form, and in various forms intermediate between these extremes.
Various physical and chemical treatments have been applied to polymers in order to effect conformational and chemical changes resulting in changes in measureable properties of the polymer (e.g., molecular weight, melting point, viscosity, acid number, etc.). Although polymers can be reacted with acids and other chemicals, they are commonly subjected to oxidation and/or elevated temperature treatments to produce various degradative effects. Commonly, thermal/oxidation treatments are facilitated by catalysts and increased pressure.
Among the various olefin polymers, the oxidation of paraffin wax has been extensively studied and provides a representative paradigm. The paraffin wax starting material can be produced from natural sources (petroleum or lignite) or can be produced synthetically (Fischer-Tropsch process). The oxidation of these waxes either by means of chemicals or through the use of air or other oxygen-containing atmospheres results in the formation of water-soluble fatty acids which have been used for the manufacture of soaps and various fatty acid esters. The oxidations also commonly produce undesirable degradation by-products having dark colors and/or unpleasant smells.
Various researchers during the past century have identified the importance of various factors affecting the oxidation of paraffin wax. See, for example, Asinger, F., Paraffins, Chemistry and Technology, 1968, pages 583-604. Temperature, catalysts, pressure, and air or oxygen distribution have been found to be some of the significant variables.
For example, by using low reaction temperatures and short reaction times it has been found that the quality of the final product can generally be improved by reducing undesirable by-products (e.g., dark oxidates, alcohols, ketones, aldehydes, esters, lactones, peroxides, low molecular weight fatty acids, etc.). A low reaction temperature obviates over-oxidized substances and keeps a good product color. However, it also markedly extends the reaction time and attenuates the utilization of atmospheric oxygen.
To reduce reaction times, catalysts, increased temperatures, and, in some cases, increased pressures resulting in increased amounts of dissolved oxygen, have been used. Significantly, however, with regard to the present invention, it has been generally observed that from a commercial point of view, the oxidation process is unacceptably long even at elevated temperatures and pressures without the aid of catalysts.
Various patents have addressed the problem of degrading or depolymerizing polymers. For example, the thermal degradation or thermal oxidative degradation of polyolefins is described in U.S. Pat. Nos. 3,110,708, 3,519,588, and 3,898,209, including references to pressure, molten preparations, catalysts, and solvents. U.S. Pat. No. 3,700,639 describes the thermal degradation in air of highly crystalline copolymers of styrene and alpha-olefins. The thermal oxidative degradation of solid and molten polyethylene is described in U.S. Pat. Nos. 2,928,797, 3,177,193, 3,201,381, and 3,655,834. The thermal oxidative treatment of polypropylene is presented in U.S. Pat. Nos. 2,911,384, 3,110,708, 3,130,186, 3,563,972, 3,383,375, and 3,898,209, including use, in part, of solvents, catalysts, and increased pressure to produce products of lower molecular weight than the starting materials.
The use of air streams or other oxygen-containing atmospheres in connection with the thermal degradation of polymers is described in a number of patents. For example, U.S. Pat. No. 4,145,526 describes the use of a stream of gas to fluidize a bed of polymer particles composed of hydroxycarboxylic homopolymers or copolymers. U.S. Pat. No. 3,227,703 describes the use of an air stream during a process of thermally degrading odorous solid polyolefins (i.e. granular polyethylene). The use of oxygen/air bubbles or air sparging in connection with the oxidation of particular polymers is discussed in U.S. Pat. Nos. 2,828,296, 3,160,621, 3,232,917, and 4,145,493. In direct contrast to the present invention, the teaching of these prior art references is that the use of oxygen/air bubbles or sparging alone results in slow reaction times.